I guess the obvious thing to say is to consider seeing your doctor. If you are a young man and you are having troubles maintaining an erection, there might either be a (hopefully minor) problem or remedies can be suggested to you to help you maintain it.

My second thought is that maybe the more times you've had problems, perhaps the more it preys on your mind, even subconsciously, and that's why you've been continuing to have issues, even though you find your girlfriend sexually attractive. If you keep losing your erections at the same point (like, as you say, changing positions) do you find you are thinking "oh dear, I could lose my erection here" or "this is the point when it usually happens..." because that could be causing you to become perhaps a bit stressed and end up fulfilling the same pattern again. Even if you don't consciously think that 'this is the point' your body may subconsciously recognise it and tense or release hormones/chemicals that cause you to lose that erection.